Japanese Unexamined Patent Application Laid Open 2004-097917 discloses the method of forming a multi-layer paint film using an aqueous first colored paint (A), an aqueous second colored paint (B) and a clear paint (C) in which a paint film comprising three layers is obtained by means of a three-coat one-bake system comprising (1) a process in which the aqueous first colored paint (A) is applied, being a process in which an aqueous colored paint of solid fraction from 45 to 90 wt % of which the structural components include in 100 parts by weight of resin solid fraction comprising urethane emulsion (a) and other components (b) from 50 to 90 wt % of urethane emulsion (a) and from 10 to 50 parts by weight of other components (b), and in which from 10 to 200 parts by weight of pigment component (c) have been compounded, (2) a process in which setting is carried out at normal temperature and the paint film viscosity obtained by applying the aqueous first colored paint (A) is set to 1×103 Pa·sec (shear rate 0.1 sec−1), (3) a process in which the aqueous second colored paint (B) of solid fraction from 15 to 50 wt % is applied, (4) a process in which preliminary heating is carried out and (5) a process in which the clear paint (C) is applied and the paint film comprising three layers is baked and dried simultaneously is known as a method of forming an aqueous 3C1B (three-coat one-bake) multi-layer paint film which has a good finish with no running, unevenness or layer-mixing. However with this method, at the time of the second layer application and in the preliminary heating process, dissolution and swelling of the first layer arise as a result of the water and amine material which is included in the second layer and there is a problem in that a satisfactory paint film appearance is not obtained.
Japanese Unexamined Patent Application Laid Open 2004-337670 provides a method for forming a multi-layer paint film in which an aqueous mid-coat paint (1), an aqueous base paint (2) and a clear paint (3) are applied sequentially in a wet-on-wet system and the multi-layer paint film obtained is baked and hardened all at the same time which is characterized in that the aforementioned aqueous mid-coat paint (1) includes an acrylic emulsion of average particle diameter from 0.05 to 10 μm, urethane emulsion of average particle diameter from 0.01 to 1 μm and hardening agent, the average particle diameter of the acrylic emulsion is the same as or greater than the average particle diameter of the urethane emulsion, and the particle number ratio of the acrylic emulsion/urethane emulsion is from 1/0.1 to 1/500 is known as a method in which, with a 3C1B painting system, the impact resistance and paint film appearance are improved using an aqueous paint for the mid-coat and the base. However, with this method the improvement in appearance is inadequate and there is a weakness in that the problem of wrinkling arises.
Japanese Unexamined Patent Application Laid Open 2004-358462 discloses a method for forming a multi-layer paint film which includes (1) a process in which there is provided an object which is to be painted on which an electro-deposited paint film has been formed; (2) a process in which an aqueous mid-coat paint is applied over the electro-deposited paint film and a mid-coat paint film is formed; (3) a process in which an aqueous base-paint and a clear paint are applied sequentially with a wet-on-wet system on the mid-coat paint without hardening the mid-coat paint and a base-paint film and a clear paint film are formed and (4) a process in which the mid-coat paint film, the base-paint film and the clear paint film are baked and hardened simultaneously which is characterized in that the mid-coat paint film which is formed from said aqueous mid-coat paint has a paint film water uptake of not more than 10% and a paint film water discharge rate of not more than 5%, and said aqueous mid-coat paint includes acrylic resin emulsion which has a glass transition temperature of from −50 to 20° C., an acid value of from 2 to 60 mgKOH/g and a hydroxyl group value of from 10 to 120 mgKOH/g, a urethane resin emulsion which has an acid value of from 5 to 50 mgKOH/g, and hardening agent, is known as a method of forming a multi-layer paint film with which mixing of the mid-coat paint film and the base-paint film is prevented effectively and which has excellent surface smoothness. However, with this method the mid-coat paint film is swelled by the amine which is included in the aqueous base-paint and so there is a weakness in that a satisfactory paint film appearance is not obtained.
Japanese Unexamined Patent Application Laid Open 2003-064149 provides a water dispersed polyisocyanate compositions which contain polyisocyanate (A) and polyurethane resin (B) which has nonionic groups in side chains are known as aqueous hardenable compositions which provide water dispersible polyisocyanate compositions which have excellent dispersibility in water, stability in respect of water and compatibility with aqueous resins which have active-hydrogen containing groups and have a long use time, and which have excellent hardening properties and provide hardened materials which have excellent appearance, water resistance and hardness (for example, see Patent Citation 4). However, here there is no description of a three-coat one-bake method of forming a multi-layer paint film, and with paints in which this composition is used there is a weakness in that the intended excellent paint film appearance cannot be obtained and also in that the storage stability is unsatisfactory.
Furthermore, International laid Open Specification WO 2003/039767 provides a method of forming a multi-layer paint film in which a cationic electro-deposition paint (a) is painted on the object which is to be painted, heated and hardened, then a water-based thermoplastic chip-resistant paint (b) is applied, after adjusting the solid fraction content in the paint film to 40% or above a water-based mid-coat paint (c) is applied, and the paint films of both the paint (b) and the paint (c) are heated and hardened and then the top-coat paint (d) is applied and hardened in which the water-based thermoplastic chipping-resistant paint (b) is a paint which includes a polyurethane emulsion which has been obtained by chain-extending a carboxyl group containing urethane pre-polymer in the presence of an aqueous medium and said polyurethane emulsion is an auto-emulsification type polyurethane emulsion which has been obtained mixing a urethane pre-polymer which has been obtained by reacting (i) aliphatic and/or alicyclic diisocyanate, (ii) polyether diol and/or polyester diol of number average molecular weight from 500 to 5000, (iii) low molecular weight polyhydroxyl compound and (iv) dimethylolalkanoic acid in proportions such that the NCO/OH equivalent ratio is within the range from 1.1 to 1.9 is water-extended and emulsified by mixing with water after or during neutralization with tertiary amine is known for a multi-layer paint film which has excellent chipping resistance and corrosion resistance (for example, see Patent Citation 5). However, with this composition baking is carried out after coating the aqueous mid-coat paint and there is a weakness in that the energy saving and more efficient painting operability which are the original aims of the three-coat one-bake multi-layer paint film forming method are inadequate.